Generally, a stainless steel excellent in oxidation resistance and corrosion resistance has been used for a member used in an exhaust gas flow passage of a vehicle. Particularly, with regard to an upper stream member in the exhaust gas flow passage in which a working temperature is high, for example, members for exhaust systems such as an exhaust gas manifold, a catalytic converter, a front pipe, and the like, a high-temperature exhaust gas is discharged from an engine passes therethrough; and therefore, various characteristics such as high oxidation resistance, high-temperature strength, and heat-resistant fatigue characteristics are demanded.
In the related art, as disclosed in Patent Documents 1 to 6, a material SUS429 (14Cr—Nb steel) in which Nb is added to increase the high-temperature strength, a material SUS444 (19Cr—Nb—Mo steel) in which Mo is added together with Nb, and the like have been used for the above-described members for the vehicle exhaust system. In all of the materials, addition of Nb is assumed. This is to be because the high-temperature strength is increased by solid-solution strengthening or precipitation strengthening due to Nb or Mo.
The SUS429 steel is a stainless steel of a relatively low alloy; and therefore, workability is excellent. However, the usage environment thereof is limited to a portion in which the maximum achieving temperature is in a range of 750° C. or lower. In addition, the SUS444 steel has a strong high-temperature strength that may withstand the maximum achieving temperature of 850° C.; however, there is a problem in that workability is inferior to the SUS429 steel.
Therefore, in recent years, as disclosed in Patent Documents 7 and 8, as an intermediate grade material between the SUS429 steel and the SUS444 steel, a composite addition steel of Nb—Cu and Nb—Ti—Cu has been developed in which the heat resistance that is the problem of the SUS429 steel is improved and a decrease in workability is reduced. Characteristics of the composite addition steel are as follows. The high-temperature strength is increased by utilizing the solid-solution strengthening and the precipitation strengthening of Cu, and workability is improved by decreasing an added amount of Nb or Mo compared to SUS444.
Here, the precipitation strengthening of Cu as described above is exhibited in the middle of the usage under a high-working-temperature environment in the members for the exhaust system and the like after processing the composite addition steel, and when being processed into the members for the exhaust system, Cu is generally solutionized (solid-solubilized). Therefore, the Cu-added steel is advantageous in workability compared to the Nb-added steel in which precipitates are difficult to be solutionized completely. In addition, Mo is easy to be solutionized completely in the production process as is the case with Cu. However, solid-solution strengthening ability of Mo at an ordinary temperature is higher than that of Cu, and workability of Mo is lower than that of Cu. Furthermore, Mo and Nb are elements that are more expensive than Cu; and therefore, substitution by Cu leads to cost reduction of an alloy.
Generally, the ferritic stainless steel has low toughness compared to a common steel. Therefore, when a hot-rolled coil is uncoiled, and the resultant thin sheet is passed through respective processes such as cold rolling, pickling, and annealing, cold cracking such as edge cracking and sheet fracture may occur. In view of the circumstance, optimization of hot-rolling and coiling conditions is performed so as to secure the toughness of the hot-rolled sheet. In addition, in a stainless steel containing Nb or Mo, the toughness of the hot-rolled sheet decreases due to precipitates of which a precipitation noze is in a range of 650° C. to 700° C., for example, a Laves phase (Fe2Nb, Fe2Mo) or Fe3Nb3C; and therefore, coiling is generally performed at a temperature of 550° C. or lower.
In addition, even in a steel in which 1% or more of Cu is added, there is a problem in that the toughness decreases due to the precipitates of Cu.
For example, Patent Document 9 discloses a technology of improving toughness by setting the coiling temperature to be in a range of 550° C. or lower with regard to a Cu-added non-oriented electrical steel sheet. In addition, in a specific example, it is disclosed that the toughness is improved when coiling is performed at 500° C., 520° C., or 540° C.
On the other hand, with regard to a material of the Cu-added steel, review has been made with a focus on a carbon steel.
For example, Non-Patent Document 1 discloses an effect of Cu on material characteristics of a Ti-added ultralow-carbon steel sheet. Specifically, Non-Patent Document 1 discloses that with regard to a steel containing 1.3% of Cu, in the case where a coiling temperature of a hot-rolled sheet is set to R. T. (room temperature), the Lankford value (r value) increases to the highest degree, and the r value decreases in the order of the case of coiling at 550° C. and the case of coiling at 780° C. In addition, with regard to a texture at that point of time, an effect of the coiling temperature on a texture in a (222) orientation is not recognized; however, amounts of textures in (211) and (200) orientations become the lowest values in the case where the coiling temperature is set to R.T.
In order to improve the above-described characteristics, a ferritic stainless steel sheet in which elements such as Cr and Mo are added as a main component has been developed. However, as described above, in recent years, Cu-added steel sheet has been developed.
Patent Document 10 discloses a cold-rolled stainless steel sheet for components of a vehicle exhaust system. In the cold-rolled stainless steel sheet, 1% by weight or more of Cu is added so as to utilize precipitation strengthening due to Cu precipitates in an intermediate temperature range and to utilize solid-solution strengthening due to solid-solubilized Cu in a high temperature range.
However, generally, when producing a steel sheet in which a large amount of Cu is added, cold cracking may occur in some cases; and therefore, deterioration in productivity caused by the cold cracking becomes problematic. Meanwhile, the cold cracking represents a phenomenon in which edge cracking or sheet fracture occurs due to deficiency in toughness of a hot-rolled coil when a steel sheet is allowed to pass through a continuous pickling line or a continuous annealing and pickling line after the hot-rolled coil is uncoiled.
Patent Document 11 discloses a technology with respect to a cold-rolled annealed sheet of a ferritic stainless steel containing 2.0% by mass or less of Cu; however, the toughness of the hot-rolled sheet is not implied. On the other hand, Patent Document 11 discloses a technology in which water cooling is performed immediately after hot rolling so as to suppress generation of precipitates in a cold-rolled sheet, and then the coiling treatment is performed.
However, Patent Document 11 does not disclose a coiling temperature and the like. In addition, it is difficult to cool to the vicinity of room temperature after hot rolling in light of a capability aspect of cooling equipment. In addition, a termination temperature of the water cooling is unclear, and practically applicable conditions are also unclear.
As a ferritic stainless steel in which the toughness of the hot-rolled steel is problematic, steel types in which the content of Cr is large or steel types in which Al is added may be exemplified, and as methods (techniques) for solving the toughness of these hot-rolled sheets, Patent Documents 12 to 14 are known.
As a technology of improving a toughness value of a hot-rolled sheet of steel types in which 25% by weight to 35% by weight of Cr is added, Patent Document 12 discloses a technology in which coiling is performed at a temperature of 400° C. to 600° C., and immediately after the coiling, rapid cooling is performed at a cooling rate higher than water cooling.
In addition, Patent Document 13 discloses a technology in which a ferritic stainless steel containing 3% by weight to 7% by weight of Al is subjected to rapid water-cooling after coiling.
Patent Document 14 discloses a method in which a steel sheet is coiled to have a coiled shape under a condition where the coiling temperature is set to be in a range of 550° C. to 650° C., and then the coil is immersed in a water bath within 3 hours from the coiling.